US 4410377 A
Improved compositions are described for solid propellant NF3 -F2as generators using (NF4)2 TiF6 and clinkering agents derived from LiF, KF, and NaF, either alone or in mixtures.
1. Solid propellant NF3 -F2 gas generator composition comprising (NF4)2 TiF6 with LiF as a clinker forming agent.
2. Solid propellant NF3 -F2 gas generator compositions according to claim 1 wherein the mole ratio of LiF to (NF4)2 TiF6 is from 1 to 2.4.
3. Solid propellant NF3 -F2 gas generator composition comprising (NF4)2 TiF6 with NaF as a clinker forming agent.
4. Solid propellant NF3 -F2 gas generator compositions according to claim 3 wherein the mole ratio of NaF to (NF4)2 TiF6 is from 1 to 2.4.
5. Solid propellant NF3 -F2 gas generator compositions comprising (NF4)2 TiF6 with mixtures of LiF with heavier alkali metal fluorides MF, where M is selected from Na and K, as a clinker forming agent.
6. Solid propellant NF3 -F2 gas generator compositions according to claim 5 wherein the mole ratio of LiF and MF to (NF4)2 TiF6 is from 1 to 2.4.
The invention described herein was made in the course of or under a contract or subcontract thereunder with the Government and may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalties thereon.
1. Field of the Invention
This invention relates to improved compositions for solid propellant NF3 -F2 gas generators, useful, for example, in chemical HF-DF lasers.
2. Description of Prior Art
NF4 + salts are the key ingredients for solid propellant NF3 -F2 gas generators, as shown by D. Pilipovich in U.S. Pat. No. 3,963,542. These propellants consist of a highly over-oxidized grain using NF4 + salts as the oxidizer. Burning these propellants with a small amount of fuel, such as aluminum powder, generates sufficient heat to thermally dissociate the bulk of the oxidizer. This is shown for NF4 BF4 in the following equation:
NF4 BF4 →NF3 +F2 +BF3
As can be seen from the equation the gaseous combustion products contain the volatile Lewis acid BF3. This disadvantage of a volatile Lewis acid byproduct is shared by most known NF4 + compositions. These volatile Lewis acids possess a relatively high-molecular weight and a low γ value (γ=Cpi /Cvi), relative to the preferred diluent helium and frequently act as a deactivator for the chemical HF-DF laser. Consequently, these volatile Lewis acids must be removed from the generated gas prior to its use in an efficient chemical laser. Based on the state of the art, heretofore, this would be achieved by adding a clinker forming agent, such as KF, to the solid propellant formulation. The function of this additive served to convert the volatile Lewis acid, such as BF3, to a non-volatile salt as shown by the following equation:
Since the addition of KF significantly increases the weight of the formulation while the amount of evolved NF3 and F2 remains the same, the yield of NF3 and F2 per pound of formulation is decreased. For NF4 BF4 based formulations, the replacement of KF by the lighter alkali metal fluorides NaF or LiF would theoretically improve the obtainable NF3 -F2 yield, but was found to be impractical due to the insufficient thermal stability of NaBF4 and LiBF4, resulting in incomplete clinkering of the BF3.
The use of self-clinkering formulations based on (NF4)2 TiF6 has previously been proposed by Christe et al (U.S. Pat. No. 4,152,406) as means of increasing the theoretically obtainable NF3 -F2 yield relative to that of the state of the art NF4 BF4.1.2KF formulation. However, test firings of (NF4)2 TiF6 based formulations showed that the relatively high volatility of TiF4 (boiling point of 284° C.) resulted in the deposition of TiF4 throughtout the whole gas generator system. To eliminate TiF4 from the generated gas, KF had to be added to the (NF4)2 TiF6 based formulations. This necessary KF addition lowered the theoretically obtainable NF3 -F2 yield to a value of 39.5 weight % which is comparable to that of 38.5 weight % of the KF clinkered NF4 BF4 system and thus eliminated most of the improvement offered by the use of (NF4)2 TiF6.
The above described problem of obtaining an (NF4)2 TiF6 based formulation of significantly higher performance than that offered by the state of the art NF4 BF4.1.2KF system is overcome by the present invention. We have found that, contrary to NF4 BF4, (NF4)2 TiF6 forms thermally stable clinkers with the lighter alkali metal fluorides NaF and LiF and that less than stoichiometric amounts of these alkali metal fluorides are required for the formation of a stable clinker due to the ability of TiF4 to form polytitanate anions. The improvements resulting from this concept are summarized in Table 1.
TABLE I______________________________________Theoretical Yields of Usable FluorineSystem F Yield (Weight %)______________________________________NF4 BF4.1.2KF 38.7(NF4)2 TiF6.2.4KF 39.5(NF4)2 TiF6.2.4NaF 42.9(NF4)2 TiF6.2.4LiF 47.0(NF4)2 TiF6.1.2NaF 48.4(NF4)2 TiF6.1.2LiF 50.9______________________________________
Accordingly, it is an object of the present invention to provide high performing solid propellant NF3 -F2 gas generator formulations based on (NF4)2 TiF6 and low molecular weight alkali metal fluorides.
This and other objects and features of the present invention will be apparent from the following examples. It is understood, however, that these examples are merely illustrative of the invention and should not be considered as limiting the invention in any sense.
A formulation containing 81.44 weight % (NF4)2 TiF6, 14.71 weight % LiF and 3.83 weight % Al was fired in a typical gas generater. Smooth burning was observed. Disassembly of the generator after completion of the test showed that the desired clinker had formed in the combustion chamber with essentially no TiF4 deposited in the coolant pack section of the generator. The clinker was shown by chemical analysis and vibrational spectroscopy to consist mainly of M2 TiF6 where M is Li.
A formulation containing 85.2 weight % (NF4)2 TiF6, 3.8 weight % LiF, 8.5 weight % KF and 2.5 weight % Al was fired in a typical gas generator. Again essentially complete clinkering of the TiF4 was observed and the clinker was shown by elemental and spectroscopic analysis to consist mainly of M2 Ti2 F10 where M is Li and/or K.
A formulation containing 86.0 weight % (NF4)2 TiF6, 7.3 weight % NaF, 6.7 weight % KF and 2.5 weight % Al was fired in a typical gas generator. Again essentially complete clinkering of the TiF4 was observed and the clinker was shown by elemental and spectroscopic analyses to consist mainly of M2 Ti2 F10 where M is Na and/or K.
Obviously, numerous variations and modifications may be made without departing from the present invention. Accordingly, it should be clearly understood that the forms of the present invention described above are illustrative only and are not intended to limit the scope of the present invention.